I. Technical Field
The following description relates generally to wireless communications, and more particularly to mapping resource elements for transmission of a reference signal over a channel.
II. Relevant Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the Ns independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
Wireless systems often use reference signals in the uplink (UL) or in the downlink (DL) direction for channel characterization and performance improvement. In conventional wireless systems, such as the Release-8 version of the Long Term Evolution (LTE) specification (Rel-8), certain algorithms are defined for selecting time/frequency locations used for transmissions of the reference signals in UL or DL direction. However, these conventional algorithms are limited to a single layer transmission of certain reference signals, such as the user equipment reference signal (UE-RS). With the introduction of multi-user multiple-input, multiple-output (MU-MIMO) wireless systems, such as LTE Release 9 (Rel-9), there is a need to extend the reference signal framework to support the increased number of antennas and beamforming in multiple layers of transmission.